1. Field of the Invention
The present invention relates to an automatic speed reduction ratio switching apparatus for automatically switching a speed reduction ratio between an input shaft connected to a rotary drive source and an output shaft connected to an actuating member.
2. Description of the Related Art
Heretofore, as a means for transporting workpieces and the like, an electric actuator has been widely used, in which a feed screw is rotatably driven by a rotary drive force of a rotary drive source such as a motor or the like, for thereby displacing a slider (actuating member) that transports a workpiece.
This type of actuator is equipped with an input shaft connected to the rotary drive source, and an output shaft connected to the actuating member, and further includes a planetary mechanism, which is capable of changing a speed reduction ratio between the input shaft and the output shaft.
As equipment that is provided with such a planetary mechanism, the automatic speed reduction ratio switching apparatus disclosed in Japanese Laid-Open Patent Publication No. 2007-092982 is known. This automatic speed reduction ratio switching apparatus has a sun gear made up of a helical gear, an internal gear made up of a helical gear, and planetary gears made up of helical gears. The apparatus further comprises a planetary gear mechanism supporting the planetary gears axially for enabling rotation thereof and including a carrier that rotates integrally accompanying revolution of the planetary gears, a viscous resistance medium disposed between an inner member of the carrier that is inserted into the internal gear, the planetary gears, and the internal gear, which induces generation of a thrust force between the internal gear and the planetary gears, and a control means, which causes parallel displacement of the internal gear in the direction of the input shaft or the direction of the output shaft by means of the thrust force in accordance with an increase and decrease in the output load.
According to the aforementioned Japanese Laid-Open Patent Publication No. 2007-092982, by utilizing the viscous resistance medium, in accordance with an increase and decrease in the output load, the internal gear is displaced in parallel in the direction of the input shaft or the direction of the output shaft as a result of the thrust force. Owing thereto, rotational movement of the internal gear is controlled, whereby switching of the speed reduction ratio is performed automatically by switching between rotation and revolution of the planetary gears.
However, since grease, for example, is used to provide the viscous resistance, the viscosity (resistance value) of the grease tends to vary due to changes in temperature, and there is also concern that leakage of the grease may be brought about. Thus, a problem exists in that torque characteristics of the electric actuator are subject to variation.